Chaning the Logical network of a VM network

I have 3 x 2012 R2 hyper-V hosts in the same data centre.

They are stand-alone and some virtual machines use Hyper-V replica.

Each host has 4 NICs, 1 for management (not available to Hyper-V), 1 for "external" services (physically plugged into DMZ switch) and 2 for internal services.

In SCVMM, under the "VM Networks" node, I can see External, Internal1 and Internal2, each are associated with a logical network that auto-created by SCVMM.

I would simply like to use "External" and "internal" as my logical networks. I have created these logical networks, associated the IP ranges, but am unable to change association in the "VM networks" area.

What am I doing wrong ?

Thanks.